In the prior art, an electrostatic chuck known as an electrostatic adsorption wafer chuck is used to hold a substrate during the manufacture of a semiconductor integrated circuit. As an insulating layer of this electrostatic chuck, alumina, aluminum nitride, a ceramic material such as boron nitride, an organic resin such as a polyimide, or a elastic body such as silicone rubber, has been proposed.
In an ion implantation process, it is required to perform ion implantation smoothly wherein the temperature rise of a wafer due to the heat of an ion beam is suppressed, the temperature of the wafer is kept uniform and constant, and there is no heat damage to the wafer. For this purpose, in such an ion implantation process, a platen mechanism for cooling the wafer, wherein a cooling mechanism which circulated a cooling chiller in a cooling channel formed on the back side of the electrostatic chuck or in a plinth, was provided.
For example, it is known that an electrostatic chuck using a ceramic insulating layer has excellent durability and high thermal conductivity. However, since the ceramic insulating layer has a high hardness, it has poor adhesion to a wafer with surface unevenness, and since the contact thermal resistance increases as a result, sufficient heat dissipation is not obtained. Hence, in order to promote heat dissipation, a gas cooling method is adopted wherein an inert gas such as helium is circulated between the wafer and the insulating layer. However, in this method, not only is it necessary to perform fine machining to form a groove in the insulating layer surface through which the inert gas can be passed, but an apparatus is required to supply the inert gas, and since the structure of the electrostatic chuck becomes more complex, there is a disadvantage of increasing the manufacturing cost of the electrostatic chuck. Moreover, since the insulating layer is hard and particles are generated when the wafer is brought into contact and rubs against it, if the wafer is processed with particles still adhering to its back side, defects may occur during the fine machining of the wafer.
Although an electrostatic chuck using an insulating layer made from polyimide can be manufactured easily and is also economical, there is a problem that since it has a low thermal conductivity and is hard, it has a large contact thermal resistance, and heat dissipation properties are poor. Also, as in the case of an electrostatic chuck using a ceramic insulating layer, there is the problem that particles are generated.
An electrostatic chuck has been proposed comprising a first insulating film of a thermally conductive silicone rubber prepreg obtained by impregnating glass cloth with silicone rubber on a metal plate, a copper pattern functioning as an electrode on the first insulating film, and a second insulating layer of silicone rubber formed on the copper pattern (JP-A 59-64245). Since this electrostatic chuck uses silicone rubber which is an elastic body as an insulating layer, its contact thermal resistance is relatively small and heat dissipation properties are good, so it dissipates heat efficiently and tends to keep the temperature of a wafer uniform. Moreover, since silicone rubber is soft, even if it comes in contact with a wafer, it does not easily generate particles, and a low level of particulate generation can be realized. However, in recent years, still higher heat dissipation properties and lower particulate levels are being demanded, and in this respect, the performance of this electrostatic chuck is inadequate.
For this reason, another electrostatic chuck was proposed comprising a first insulating layer of thermally conductive silicone rubber having a thermal conductivity of 0.2 W/mK or more on a metal substrate, an electrically conducting pattern formed as an electrode on this first insulating layer, and a second insulating layer of thermally conductive silicone rubber having a thermal conductivity of 0.2 W/mK or more, a hardness of 85 or less, and a surface roughness of 5 μm or less formed on this electrically conducting pattern. This chuck has superior heat dissipation properties, and is able to keep the wafer temperature uniform and constant to a high degree of precision (JP-A 9-298233). However, in the case of this electrostatic chuck, since sputter metal is introduced, there is concern about insulation abnormalities arising in the second insulating layer. Further, in this electrostatic chuck, there is also concern that thermally conductive fillers such as alumina and boron nitride which are added to impart thermal conductivity to the silicone rubber may drop out, and generate the undesirable particles mentioned hereinabove.
Still another electrostatic chuck has been proposed, comprising a first insulating layer of thermally conductive silicone rubber having a thermal conductivity of 0.2 W/mK or more formed directly or via an adhesive layer on a metal substrate, an electrically conducting pattern formed directly or via an adhesive layer on this first insulating layer, a second insulating layer of an insulating polyimide film formed directly or via an adhesive layer on this electrically conducting pattern, and a third insulating layer formed directly or via an adhesive layer on this second insulating layer, this third insulating layer being of silicone rubber having a thermal conductivity of 0.2 W/mK or more, a hardness of 85 or less and a surface roughness of 5 μm or less. Since it has excellent cooling performance and insulating properties, it has been proposed as suitable for holding a substrate during the manufacture of a semiconductor integrated circuit (JP-A 2006-165160). Since this electrostatic chuck uses an insulating polyimide film for the second insulating layer, there is little possibility of causing insulation abnormalities. However, if the specific surface area of a thermally conductive filler added to the silicone rubber of the third insulating layer is small, there is concern that it will be only weakly integrated with the silicone polymer, and since it has no active sites, which can bind to the silicone polymer, on the surface, particles may be generated. Moreover, the thermally conductive filler is hard and may abrase the wafer by rubbing, which may also generate more particles.